docs: def decode_tag(stream):
      """Decode a tag value from a serialized stream.
  
      :param stream: Source data stream
      :type stream: io.BytesIO
      :returns: Decoded tag
      :rtype: bytes
      """
      (reserved, tag) = unpack_value(">cc", stream)
  
      if reserved != b"\x00":
          raise DeserializationError("Invalid tag: reserved byte is not null")
  
      return tag